When you are signed in to the EOAS Web Mail interface (https://exchange.eoas.ubc.ca), click on the avatar icon (blank head, or a picture), at the top right. Select "Open another mailbox" from the drop down menu.
In the prompt that appears, start typing the full name of the mailbox you with to open, not the email name. For example, if you were trying to open the shared mail box for "Study Group" and the email address is "firstname.lastname@example.org", you must type "Study Group" into the prompt box. If the name does not autofill, type it in and select "Search Directory" to find the mailbox.
One you have selected the correct mailbox, click Open to be brought to a new window with the other mailbox.
If you know you will be accessing the shared mailbox often, you can bookmark the URL for future use.